High powered multichannel communication satellites for land and sea mobile communications experience a source of interference called passive intermodulation (PIM). The basic PIM phenomenon is caused by currents flowing in components with non-linear voltage-current behavior. These components then radiate and the resultant signals are picked up as noise in the system. These non-linear components can generate harmonic noise in a single carrier system, intermodulation in a multiple carrier system, and even intermodulation in a single carrier system where there is a pick-up in the system from other nearby radiations.
In most early communication systems, the multitude of noise frequencies was not a significant concern. The amplitude of the noise was several orders of magnitude lower than the signal. Space communication systems, however, require the coexistence of high power transmissions and low power receptions, often in the same radio frequency (RF) hardware. With the trend toward higher power, wider bandwidths and greater receiver sensitivity, the susceptibility of new mobile communication satellites to PIM problems is increasing.
The phenomenon of passive intermodulation is discussed, for example, in "Passive Intermodulation Product Generation In High Power Communications Satellites," Ford Aerospace & Communications Corp., 1985, and in Passive Intermodulation Interference in Communication Systems, ELECTRONICS & COMM. ENG. JOURNAL, June, 1990.
Many potential causes of PIM have been identified, but finding cures for the problem has not always been successful. Also, each spacecraft design is unique and has its own set of problems. Some general solutions to the problem involve quality workmanship, thorough testing procedures,. and proper choice of components and materials.
In order to provide protection from some PIM signals (as well as other environmental factors), communication satellites and other spacecraft typically employ protective blankets with PIM shields over the main bodies of the spacecraft. These protective blankets generally utilize conductive foils and thin film materials, or carbon-filled and thin film materials. PIM protection is also needed on auxiliary and/or protruding components, such as antennas and arrays.
Most satellites utilize a pair of antennas and may include a reflector mesh for transmitting and/or receiving signals from ground stations. The mesh is stretched and mounted over open frames. The mesh is positioned on the inside concave surfaces of the parabolic reflectors. Typically, to minimize interference, a transmitting antenna is positioned on one side of the spacecraft body and a receiver antenna is positioned on the opposite side. Satellites with a single dualpurpose antenna are typically more efficient and save significant expense, hardware and weight. Single antennas on spacecrafts also have the capability to use a common feed, filtering system, reflector and boom, which also saves weight and expense. Single antenna spacecraft are not favored however, due to potential interference problems, typically caused by PIM.
Antennas on satellites and other spacecraft normally function in the range of 100 MHz to 100 GHz, although the missions may vary widely for commercial, scientific, or military purposes. The antenna reflectors typically range up to 30-50 feet or more in diameter. These large antennas are designed to be foldable for storage and transport into orbit and deployable to their full size once the spacecraft reaches its destination. Where mesh reflectors are utilized, a flexible mesh material is typically stretched over a rib or other type of structure which has a parabolic dish shape. In the past, the mesh for the antennas has been made from a variety of materials, including metallized materials, fiberglass, polyester materials, synthetic materials, fibrous metal materials, and the like, and combinations thereof. Metallic meshes are discussed, for example, in Levy et al., "Metallic Meshes for Deployable Spacecraft Antennas," SAMPLE JOURNAL (May/June 1973).